HomeMy WebLinkAboutSubsoil StudyK+rf i;ffififfiürn$yå**
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5020 Counf; Road 154
Glenwood Springs, CO 81601
phone: (970) 945-7988
fax: (970) 945-84s4
email : kaglenwood@kumarusacom
wwwkuma¡usa.com
Offrce l¡cations: Denver (HQ), Parke¡, Colorado Springs, Fort Collins, Glenwood Springs, and Summit Counry, Colorado
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STIBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT 36, SPRTNG RrDGE RESERVE
HIDDEN VALLEY DRTVE
GARFTELD COUNTY, COLORADO
PROJECT NO.20-7-226
APRrL 2t,2O2O
PREPARED FOR:
ROSS AND KATHY STA}IGEL
23688 WEST LAKE VISTA AYENUE
ANTTOCH, TLLINOTS 60002
@
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY
PROPOSED CONSTRUCTION
SITE CONDITIONS
FIELD EXPLORATION
SUBSUP.FACE CONDITIONS ..
FOUNDATION BEARING CONDITIONS ..
DESIGN RECOMMENDATIONS
FOUNDATIONS
FOUNDATION AND RETAINING WALLS ...
FLOOR SLABS
UNDERDRAIN SYSTEM
SURFACE DRAINAGE....
LIMITATIONS..
FIGURE 1 - LOCATION OF EXPLORATORY BORINGS
FIGURE 2 - LOGS OF EXPLORATORY BORINGS
FIGURE 3 - LEGEND AND NOTES
FIGURE 4 and 5 - SWELL-CONSOLIDATION TEST RESULTS
TABLE 1- SUMMARY OF LABORATORY TEST RESULTS
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Kumar & Associates, lnc. o Project No. 20.7-226
PURPOSE A¡ID SCOPE OF STUDY
This report presents the results ofa subsoil study for a proposed residence to be located on
Lot36, Spring Ridge Reserve, Hidden Valley Drive, Garfield County, Colorado. The project
site is shown on Figure l. The pu{pose of the study was to develop recommendations for the
foundation design. The study was conducted in accordance with our agreement for geotechnical
engineering services to Ross and Kathy Stangel dated April 9,2A20.
A field exploration program consisting of exploratory borings \ilas conducted to obtain
information on the subsurface conditions. Samples of the subsoils and bedrock obtained during
the field exploration were tested in the laboratory to determine their classifTcation,
compressibility or swell and other engineering characteristics. The results of the field
exploration and laboratory testing were analyzedto develop recommendations for foundation
types, depths and allowable pressures for the proposed building foundation. This report
summarizes the data obtained during this study and presents our conclusions, design
recommendations and other geotechnical engineering considerations based on the proposed
construction and the subsurface conditions encountered.
PROPOSED CONSTRUCTION
The proposed residence will be a single story structure with attached garage. Ground floor will
be structural over crawlspace for the residence and slab-on-grade for the garage. Grading for the
structure is assumed to be relatively minor with cut depths between about 2 to 6 feet. We
assume relatively light foundation loadings, typical of the proposed type of construction.
If building loadings, location or grading plans change significantly from those described above,
we should be notified to re-evaluate the recommendations cont¿ined in this report.
SITE CONDITIONS
The property was vacant at the time of our field exploration. The site is vegetated with grass,
weeds and sage brush. The ground surface in the general building area slopes gently to
Kumar & Aesociates, lnc. @ Project No.20-7-226
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moderately down to the northeast ata grade of around 10%. Maroon Formation sandstone is
exposed on the hillside to the west of the lot.
FMLD EXPLORATION
The field exploration for the project was conducted on April 13,202A. Two exploratory borings
were drilled at the locations shown on Figure I to evaluate the subsurface conditions. The
borings were advanced with 4 inch diameter continuous flight augers powered by a truck-
mounted CME-458 drill rig. The borings were logged by a represent¿tive of Kumar &
Associates,Inc.
Samples of the subsoils were taken with a 2 inchl.D. spoon sampler. The sampler was driven
into the subsoils at various depths with blows from a 140 pound hammer falling 30 inches. This
test is similar to the standard penetration test described by ASTM Method D-l586. The
penetration resistance values are arrindication of the relative density or consistency of the
subsoils and hardness of the bedrock. Depths at which the samples were taken and the
penetration resistance values are shown on the Logs of Exploratory Borings, Figure 2. The
samples were returned to our laboratory for review by the project engineer and testing'
SUBSURFACE CONDITIONS
Graphic logs of the subsurface conditions encountered at the site are shown on Figure 2. The
subsoils consist of about Yz feetof topsoil overþing stiff to very stiff sandy clay to between
4 and 7 feet. Below the clay, weathered siltstone bedrock was encountered overlying hard
siltstone/sandstone bedrock to the maximum explored depth of 16 feet.
Laboratory testing performed on samples obtained from the borings included natural moisture
content and density and finer than sand size gradation analyses. Results of swell-consolidation
testing performed on relatively undisturbed drive samples of the clay and weathered siltstone,
presented on Figure 4 and5, generally indicate low compressibility under light loading and low
to moderate compressibility under additional loading after and wetting. The clay sample from
Boring I at2lrfeet showed moderate expansion when wetted under light loading. The
laboratory testing is summarizedinTable I'
Kumar & Associates, lnc. o Proiect No. 2&7-226
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No free water was encountered in the borings althe time of drilling and the subsoils and bedrock
were slightly moist.
FOUNDATION BEARING CONDITIONS
The top of bedrock slopes down to the east and will probably be encountered in the upper part of
the building excavation and transition to sandy clay in the remaining areas of the excavation.
The sandy clay soils have variable compressibilityiexpansion potential and could tend to settle or
heave mainly when they become wetted. A shallow foundation placed on the sandy clay soils or
bedrock will have a risk of differential settlement if the soils become wetted and care should be
taken in the surface and subsurface drainage around the house to keep the bearing soils dry. It
will be critical to the long term performance of the structure that the recommendations for
surface grading and subsurface drainage contained in this report be followed. Presented below
are recommendations for shallow spread footings with a risk of settlement. A low settlement risk
foundation support can be achieved by extending the bearing down into the underlying bedrock
such as by deepening the footing excavation to expose bedrock and the sub-excavated depth
backfilled with structural fill. The clay soils should be fi.rther evaluated for compressibility or
expansion potential at the time of excavation.
DESIGN RECOMMENDATIONS
FOUNDATIONS
Considering the subsurface conditions encountered in the exploratory borings and the nature of
the proposed construction, the building can be founded with spread footings bearing on the
natural soils or bedrock with the risk of differential settlement.
The design and construction criteria presented below should be observed for a spread footing
foundation system.
1) Footings placed on the undisturbed natural soils or bedrock should be designed
for an allowable bearing pressure of 2,000 psf. Based on experience, we expect
settlement of footings designed and constructed as discussed in this section will
be about 1 inch or less which could be differential between soil and bedrock
areas.
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2)The footings should have a minimum width of 16 inches for continuous walls and
2 feetfor isolated pads.
Exterior footings and footings beneath unheated areas should be provided with
adequate soil cover above their bearing elevation for frost protection. Placement
of foundations at least 36 inches below exterior grade is typically used in this
atea.
Continuous foundation walls should be heavily reinforced top and bottom to span
local anomalies, especially across any soiUbedrock contact, such as by assuming
an unsupported length of at least 14 feet. Foundation walls acting as retaining
structures should also be designed to resist lateral earth pressures as discussed in
the "Foundation and Retaining Walls" section of this report.
Topsoil and any loose or disturbed soils should be removed and the footing
bearing level extended down to the undisturbed natural soils. The exposed soils
in footing area should then be moistened and compacted. Potentially expansive
clay soils may need to be sub-excavated and replaced \Mith structural fill
compacted to at least 98% of standard Proctor density at near optimum moisture
content.
A representative ofthe geotechnical engineer should observe all footing
excavations prior to concrete placement to evaluate bearing conditions.
4)
FOUNDATION AND RETAINING WALLS
Foundation walls and retaining structures which are laterally supported and can be expected to
undergo only a slight amount of deflection should be designed for a lateral earth pressure
computed on the basis of an equivalent fluid unit weight of at least 55 pcf for backfill consisting
of the on-site fine-grained soils and well broken bedrock. Cantilevered retaining structures
which are separate from the residence and can be expected to deflect sufficiently to mobilize the
fullactive earth pressure condition should be designed for a lateral earth pressure computed on
the basis of an equivalent fluid unit weight of at least 45 pcf for backfill consisting of the on-site
fine-grained soils and well broken bedrock.
All foundation and retaining structures should be designed for appropriate hydrostatic and
surcharge pressures such as adjacent footings, traffic, construction materials and equipment. The
3)
5)
6)
Kumar & Associates, lnc. o Project No. 20-7-220
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pressures recommended above assume drained conditions behind the walls and a horizontal
backfill surface. The buildup of water behind a wall or an upward sloping backfill surface will
increase the lateral pressure imposed on a foundation wall or retaining structure. An underdrain
should be provided to prevent hydrostatic pressure buildup behind walls.
Backfill should be placed in uniform lifts and compacted to at least 90% of the maximum
standard Proctor density at a moisture content slightly above optimum. Backfill placed in
pavement and walkway areas should be compacted to at least 95Yo of themaximum standard
Proctor density. Care should be taken not to overcompact the backfill or use large equipment
near the wall, since this could cause excessive lateral pressure on the wall. Some settlernent of
deep foundation wall backfill should be expected, even if the material is placed correctly, and
could result in distress to facilities constructed on the backfill.
The lateral resistance of foundation or retaining wall footings will be a combination of the
sliding resistance of the footing on the fourdation materials and passive earth pressure against
the side of the footing. Resistance to sliding at the bottoms of the footings can be calculated
based on a coeffîcient of friction of 0.40. Passive pressure of compacted backfill against the
sides of the footings can be calculated using an equivalent fluid unit weight of 350 pcf. The
coefficient of friction and passive prossure values recommended above assume ultimate soil
strength. Suitable factors of safety should be included in the design to limit the strain which will
occur at the ultimate strength, particularly in the case of passive resistance. Fill placed against
the sides of the footings to resist lateral loads should be a nonexpansive material compacted to at
least 95% of the maximum standard Proctor density at a moisture content near optimum.
FLOOR SLABS
The natural on-site soils, exclusive of topsoil, are suitable to support lightly loaded slab-on-grade
construction with a risk of differential movement similar to that for footings. To reduce the
effects of some differential movement, floor slabs should be separated from all bearing walls and
columns with expansíon joints which allow unrestrained vertical movement. Expansive clay
soils may need to be removed and replaced with structural fill. Floor slab control joints should
be used to reduce damage due to shrinkage cracking. The requirements for joint spacing and
slab reinforcement should be established by the designer based on experience and the intended
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slab use. A minimum 4 inch layer of relatively well graded sand and gravel such as road base
should be placed beneath interior slabs for support. This material should consist of minus 2-inch
aggregate with at least 50% retained on the No. 4 sieve and less than 12% passing the No. 200
sleve.
All fill materials for support of floor slabs should be compacted to at least 95o/o of maximum
standard Proctor density at a moisture content near optimum. Required fill should consist of
imported granular soils devoid of vegetation, topsoil and oversized rock.
UNDERDRAIN SYSTEM
Although free water was not encountered during our exploration, it has been our experience in
the area and where there are clay soils and shallow bedrock that local perched groundwater can
develop during times of heavy precipitation or seasonal runoff. Frozen ground during spring
runoff can create a perched condition. We recommend below-grade construction, such as
retaining walls, be protected from wetting and hydrostatic pressure buildup by an underdrain
system.
Where installed, drains should consist of drainpipe placed in the bottom of the wall backfill
su¡rounded above the invert level with free-draining granular material. The drain should be
placed at each level of excavation and at least 1 foot below lowest adjacent finish grade and
sloped at a minimum 1% to a suitable gravity outlet. Free-draining granular material used in the
underdrain system should contain less than 2% passing the No. 200 sieve, less than 50% passing
the No. 4 sieve and have a maximum size of 2 inches. The drain gravel backfill should be at
least llz feet deep. An impervious membrane such as 20 mil PVC should be placed beneath the
drain gravel in a trough shape and attached to the foundation wall with mastic to prevent wetting
of the bearing soils unless the bearing material is bedrock or non-moisture sensitive soil.
SURFACE DRAINAGE
Proper surface grading and drainage will be critical to limiting subsurface wetting below the
building. The following drainage precautions should be observed during construction and
maintained at all times after the residence has been completed:
Kumar & Associates, lnc. @ Project No. 20-7-226
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1)Inundation of the foundation excavations and underslab areas should be avoided
during construction.
Exterior backfill should be adjusted to near optimum moisture and compacted to
at least 95% of the maximum standard Proctor density in pavement and slab areas
and to at least 90% of the maximum standard Proctor density in landscape areas.
The ground surface surrounding the exterior of the building should be sloped to
drain away from the foundation in all directions. We recommend a minimum
slope of 12 inches in the first 10 feet in unpaved areas and a minimum slope of 3
inches in the first l0 feet in paved areas. Free-draining wall backfill should be
covered with filter fabric and capped with about 2 feet of the on-site soils to
reduce surface water infiltration.
Roof downspouts and drains should discharge well beyond the limits of all
backfill.
Landscaping which requires regular heavy inigation should be located at least 10
feet from foundation walls.
2)
3)
4)
LIMITATIONS
This study has been conducted in accordance with generally accepted geotechnical engineering
principles and practices in this arcaatthis time. We make no warranty either express or implied.
The conclusions and recommendations submitted in this report are based upon the dat¿ obt¿ined
from the exploratory borings drilled at the locations indicated on Figure 1, the proposed type of
construotion and our experience in the area. Our services do not include determining the
presence, prevention or possibility of mold or other biological contaminants (MOBC) developing
in the future. If the client is concemed about MOBC, then a professional in this special field of
practice should be consulted. Our findings include interpolation and extrapolation ofthe
subsurface condifions identified at the exploratory borings and variations in the subsurface
conditions may not become evident until excavation is performed. If conditions encountered
during construction appear different from those described in this report, we should be notified so
that re-evaluation of the recommendations may be made.
5)
Kumar & Associates, lnc. @ Project No.2G7-226
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This report has besn prepared for the exclusive use by our client for design purposes. \iVe are not
responsible for technical interpretations by others of our information. As the project evolves, we
should provide continud consultation and field services during construction to review and
monitor the implementation of our recomme,lrdationso and to veriry that the recommendations
havebeen appopriately interpreted. Significant design changesmay require additional analysis
or modifications ùo the recommendations presente herein. We recommend on-site observation
of excavations and foundation bearing strata and testing of structural fitl by a representative of
the geotechnical engineer.
Respectfully Submitted,
Kxmnr & Äçso*istes' fnc"
James H. Parsons, E.I.
Reviewedby:
Steven L.
JHP¡Kac
cc: David
Brian
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20-7-226 Kumar & Associates LOCATION OF EXPLORATORY BORINGS 1Fig
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20-7-226 Kumar & Associates SWELL-CONSOLIDATION TEST RESULTS Fig. 4
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20-7-226 Kumar & Associates SWELL-CONSOLIDATION TEST RESULTS Fig. 5
l$rliffififfiffiniiyå*'"TABLE ISU|ìíMARY OF LABORATORY TEST RESULTSSandy ClaySandy ClaySandy ClayWeathered SiltstoneSOILWPE(psf)UI¡CONFINEDcol,lPREssfirESTRENGTH2t(o/.)PLASTICINDEX36ATTERBERGLIQUID Llt¡llT{o/o)74PERCENTPASSING NO.200 stEvE(%)SAI¡DGRADATION(ôGRAVEL113118108(ocfìNATURALDRYDENSffY10910.28.39.73.5&lNATURALMOISTURECONTENT52t/,5ffnDEPTH2%I2BORINGNo.